[PATCH v5 1/4] drm: Add drm_work_fence helper

[Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>]

GPU drivers often need to queue work when a dma-fence signals
because certain operations (copy_to_user, eventfd_signal, memory
allocation) cannot run in IRQ context. This pattern is currently
open-coded in multiple drivers.

Introduce drm_work_fence — an embeddable base structure that handles
the dma-fence-callback-to-workqueue pattern in one place. Drivers
embed this in their own structure and implement ops->work() for the
deferred work and ops->destroy() for cleanup.

The helper manages:
 - kref lifetime
 - dma-fence callback registration
 - workqueue dispatch on fence signal
 - safe cancellation before driver teardown

For work that additionally requires borrowing the process MM via
kthread_use_mm(), see drm_user_fence which builds on top of this.

Suggested-by: Matthew Brost <matthew.brost@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: dri-devel@lists.freedesktop.org
Cc: intel-xe@lists.freedesktop.org
Cc: amd-gfx@lists.freedesktop.org
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
---
 drivers/gpu/drm/Makefile         |   1 +
 drivers/gpu/drm/drm_work_fence.c | 195 +++++++++++++++++++++++++++++++
 include/drm/drm_work_fence.h     |  76 ++++++++++++
 3 files changed, 272 insertions(+)
 create mode 100644 drivers/gpu/drm/drm_work_fence.c
 create mode 100644 include/drm/drm_work_fence.h

diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index e97faabcd783..c5be8e80d0c8 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -72,6 +72,7 @@ drm-y := \
 	drm_vblank.o \
 	drm_vblank_work.o \
 	drm_vma_manager.o \
+	drm_work_fence.o \
 	drm_writeback.o
 drm-$(CONFIG_DRM_CLIENT) += \
 	drm_client.o \
diff --git a/drivers/gpu/drm/drm_work_fence.c b/drivers/gpu/drm/drm_work_fence.c
new file mode 100644
index 000000000000..9f6b779d0fe9
--- /dev/null
+++ b/drivers/gpu/drm/drm_work_fence.c
@@ -0,0 +1,195 @@
+// SPDX-License-Identifier: MIT
+/*
+ * Copyright © 2024 The Linux Foundation
+ *
+ * Common DRM work fence helper.
+ *
+ * When a GPU dma-fence signals, drivers often need to perform work that
+ * cannot run in IRQ context (e.g., memory allocation, copy_to_user,
+ * eventfd_signal). This helper queues a work item when a dma-fence
+ * signals, allowing that work to run safely in a workqueue context.
+ *
+ * NOTE: This helper consumes dma_fences but CANNOT implement
+ * dma_fence_ops. Work items queued here may sleep; dma_fence_ops
+ * callbacks are called under the fence spinlock and must not sleep.
+ *
+ * For work that additionally requires accessing userspace memory via
+ * kthread_use_mm(), see drm_user_fence which builds on top of this.
+ */
+
+#include <linux/workqueue.h>
+
+#include <drm/drm_work_fence.h>
+
+static void drm_work_fence_destroy(struct kref *kref)
+{
+	struct drm_work_fence *wfence =
+		container_of(kref, struct drm_work_fence, refcount);
+
+	if (wfence->fence)
+		dma_fence_put(wfence->fence);
+
+	wfence->ops->destroy(wfence);
+}
+
+/**
+ * drm_work_fence_get - Acquire a reference to a work fence
+ * @wfence: work fence
+ */
+void drm_work_fence_get(struct drm_work_fence *wfence)
+{
+	kref_get(&wfence->refcount);
+}
+EXPORT_SYMBOL_GPL(drm_work_fence_get);
+
+/**
+ * drm_work_fence_put - Release a reference to a work fence
+ * @wfence: work fence
+ */
+void drm_work_fence_put(struct drm_work_fence *wfence)
+{
+	kref_put(&wfence->refcount, drm_work_fence_destroy);
+}
+EXPORT_SYMBOL_GPL(drm_work_fence_put);
+
+static void drm_work_fence_work(struct work_struct *w)
+{
+	struct drm_work_fence *wfence =
+		container_of(w, struct drm_work_fence, work);
+
+	wfence->ops->work(wfence);
+	drm_work_fence_put(wfence);
+}
+
+static void drm_work_fence_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
+{
+	struct drm_work_fence *wfence =
+		container_of(cb, struct drm_work_fence, cb);
+
+	queue_work(wfence->wq, &wfence->work);
+	/*
+	 * Put the transferred reference from add_callback. The stored
+	 * reference in wfence->fence is released in drm_work_fence_destroy().
+	 */
+	dma_fence_put(fence);
+}
+
+/**
+ * drm_work_fence_init - Initialize a work fence
+ * @wfence: work fence to initialize
+ * @wq: workqueue to run the worker on (must be ordered if sequencing matters)
+ * @ops: driver operations
+ */
+void drm_work_fence_init(struct drm_work_fence *wfence,
+			 struct workqueue_struct *wq,
+			 const struct drm_work_fence_ops *ops)
+{
+	kref_init(&wfence->refcount);
+	wfence->wq = wq;
+	wfence->ops = ops;
+	wfence->fence = NULL;
+	INIT_WORK(&wfence->work, drm_work_fence_work);
+}
+EXPORT_SYMBOL_GPL(drm_work_fence_init);
+
+/**
+ * drm_work_fence_add_callback - Attach a work fence to a dma-fence
+ * @wfence: work fence
+ * @fence: dma-fence to watch; ownership of this reference is transferred
+ *         to the callback — caller must NOT put it afterward.
+ *
+ * When @fence signals, a work item is queued that calls ops->work().
+ * If @fence has already signaled, the work item is queued immediately.
+ *
+ * An additional reference to @fence is stored internally in @wfence to
+ * allow drm_work_fence_cancel() to be called safely without the caller
+ * needing to hold a separate fence reference.
+ *
+ * On any return value the caller's fence reference is consumed.
+ *
+ * Return: 0 on success, negative errno on error.
+ */
+int drm_work_fence_add_callback(struct drm_work_fence *wfence,
+				struct dma_fence *fence)
+{
+	int err;
+
+	drm_work_fence_get(wfence);
+	wfence->fence = dma_fence_get(fence);
+
+	err = dma_fence_add_callback(fence, &wfence->cb, drm_work_fence_cb);
+	if (err == -ENOENT) {
+		queue_work(wfence->wq, &wfence->work);
+		dma_fence_put(fence);
+		err = 0;
+	} else if (err) {
+		dma_fence_put(wfence->fence);
+		wfence->fence = NULL;
+		drm_work_fence_put(wfence);
+		dma_fence_put(fence);
+	}
+	/* on success: transferred ref goes to drm_work_fence_cb */
+
+	return err;
+}
+EXPORT_SYMBOL_GPL(drm_work_fence_add_callback);
+
+/**
+ * drm_work_fence_cancel - Cancel a pending work fence callback
+ * @wfence: work fence
+ *
+ * Attempts to remove the pending callback before driver context teardown.
+ * The caller must hold a reference to @wfence across this call.
+ *
+ * If the callback has already fired this returns false and all cleanup
+ * has been handled internally.
+ *
+ * If removal succeeds the callback reference is released internally.
+ * The caller must still release its own reference via drm_work_fence_put().
+ *
+ * This function is safe to call from atomic context as it only acquires
+ * the dma-fence spinlock internally. If the caller also needs to wait
+ * for the worker to finish, use drm_work_fence_cancel_sync() instead,
+ * which may sleep.
+ *
+ * Return: true if callback was removed, false if it had already fired.
+ */
+bool drm_work_fence_cancel(struct drm_work_fence *wfence)
+{
+	struct dma_fence *fence = wfence->fence;
+
+	if (!fence)
+		return false;
+
+	if (dma_fence_remove_callback(fence, &wfence->cb)) {
+		wfence->fence = NULL;
+		dma_fence_put(fence);  /* callback ref */
+		dma_fence_put(fence);  /* stored ref */
+		drm_work_fence_put(wfence);
+		return true;
+	}
+
+	return false;
+}
+EXPORT_SYMBOL_GPL(drm_work_fence_cancel);
+
+/**
+ * drm_work_fence_cancel_sync - Cancel callback and wait for worker to finish
+ * @wfence: work fence
+ *
+ * Calls drm_work_fence_cancel() then cancel_work_sync() to guarantee
+ * the worker has fully completed before returning.
+ *
+ * This function may sleep. Must not be called from atomic or interrupt
+ * context. Use drm_work_fence_cancel() instead when sleeping is not allowed.
+ *
+ * Drivers must call this during teardown before freeing any resources
+ * accessed by ops->work().
+ */
+void drm_work_fence_cancel_sync(struct drm_work_fence *wfence)
+{
+	drm_work_fence_cancel(wfence);
+	if (cancel_work_sync(&wfence->work))
+		drm_work_fence_put(wfence);
+}
+EXPORT_SYMBOL_GPL(drm_work_fence_cancel_sync);
diff --git a/include/drm/drm_work_fence.h b/include/drm/drm_work_fence.h
new file mode 100644
index 000000000000..4fa369f937d7
--- /dev/null
+++ b/include/drm/drm_work_fence.h
@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Copyright © 2024 The Linux Foundation
+ */
+
+#ifndef __DRM_WORK_FENCE_H__
+#define __DRM_WORK_FENCE_H__
+
+#include <linux/dma-fence.h>
+#include <linux/kref.h>
+#include <linux/workqueue.h>
+
+struct drm_work_fence;
+
+/**
+ * struct drm_work_fence_ops - driver callbacks for a DRM work fence
+ */
+struct drm_work_fence_ops {
+	/**
+	 * @work: Called from workqueue context when the dma-fence signals.
+	 * Perform any work that cannot run in IRQ context here.
+	 */
+	void (*work)(struct drm_work_fence *wfence);
+
+	/**
+	 * @destroy: Called when the last reference is dropped.
+	 * Free the containing structure here.
+	 */
+	void (*destroy)(struct drm_work_fence *wfence);
+};
+
+/**
+ * struct drm_work_fence - embeddable DRM work fence
+ *
+ * Provides a dma-fence callback that queues a work item when the fence
+ * signals, allowing work that cannot run in IRQ context to be deferred
+ * to a workqueue. Drivers embed this in their own structure.
+ *
+ * NOTE: This helper is a *consumer* of dma_fences only. It CANNOT be
+ * used to implement dma_fence_ops. dma_fence callbacks are invoked
+ * while holding the fence spinlock; work queued here may sleep
+ * (copy_to_user, kthread_use_mm, eventfd_signal) and must not be
+ * called under that spinlock.
+ *
+ * Call drm_work_fence_init() at creation and drm_work_fence_add_callback()
+ * to arm. Call drm_work_fence_cancel_sync() before driver teardown.
+ */
+struct drm_work_fence {
+	/** @refcount: Reference count. */
+	struct kref refcount;
+	/** @work: Work item queued when the dma-fence signals. */
+	struct work_struct work;
+	/** @cb: dma-fence callback. */
+	struct dma_fence_cb cb;
+	/**
+	 * @fence: Extra reference held for safe cancel(). Set during
+	 * add_callback, released in destroy().
+	 */
+	struct dma_fence *fence;
+	/** @wq: Workqueue to run @work on. */
+	struct workqueue_struct *wq;
+	/** @ops: Driver operations. */
+	const struct drm_work_fence_ops *ops;
+};
+
+void drm_work_fence_init(struct drm_work_fence *wfence,
+			 struct workqueue_struct *wq,
+			 const struct drm_work_fence_ops *ops);
+void drm_work_fence_get(struct drm_work_fence *wfence);
+void drm_work_fence_put(struct drm_work_fence *wfence);
+int drm_work_fence_add_callback(struct drm_work_fence *wfence,
+				struct dma_fence *fence);
+bool drm_work_fence_cancel(struct drm_work_fence *wfence);
+void drm_work_fence_cancel_sync(struct drm_work_fence *wfence);
+
+#endif /* __DRM_WORK_FENCE_H__ */
-- 
2.34.1